Happens sporadically, in 5-20 minute intervals; as soon as I reconnect to my school's network (sometimes requires re-authentication), it works fine until the next failure.

When plugged into ethernet, there are no connection problems.

We use Cisco Clean Access servers. The authentication takes place in the web browser and requires username and password. I don't believe there is any authentication (WPA, etc.) on the connection itself.

I have run updates, and have not experienced any problems like this on other wireless networks. I run Ubuntu 11.04.